Press plates are used in a well known process to prepare decorative laminates. These laminates are used for surfaces such as walls, table tops, furniture, doors, kitchen cabinets, countertops, flooring tiles, and the like. The decorative laminates are formed by compressing and bonding together thermosetting resin impregnated pigmented or printed decorative cellulosic surface papers to a variety of core materials, such as particle board or phenolic resin impregnated kraft paper filler sheets and the like between press plates, with the press plate imparting a surface finish to at least one side of the pressed laminate. The material layers are first placed adjacent to a stainless steel press plate. The laminate components and press plate assembly are then pressed at high pressures and elevated temperatures to fuse the laminate materials together. After final cure, a consolidated laminate sheet or board is formed. Once cooled, the laminates are separated from the press plates, which are returned to the press for use in the manufacture of subsequent laminates.
The resulting laminate surface finish is an exact mirror image replication of the press plate surface finish against Which it is pressed. Press plates are thus an important feature in the production of consistently acceptable commercial laminate surfaces, as the laminate surfaces faithfully adopt the surface finish of the press plate.
There are two general categories of press plates, polished and textured. The scope of the present invention is directed to textured press plates used to manufacture matte or suede laminate finishes. A variety of methods result in a desired textured press plate finish. A common method is chemical etching. While this is an effective method for producing the desired textures and overall finish, it is both costly and difficult to implement. Another method of texturizing includes shot peening, such as that disclosed in U.S. Pat. No. 4,076,566.
Shot peening techniques are primarily used for their mechanical benefits, as the cumulative effect of individual particle impacts on the workpiece surface cold works the article. As the shot particles are propelled or blasted at high velocities onto the metal surface, the resulting plastic flow of the metal somewhat relieves residual tensile stresses near the surface of the article by inducing compressive stresses. The mechanical benefits are improved fatigue strength and stress corrosion resistance, or if desired, reshaping of flat pieces. Two major parameters must be controlled to achieve the desired results--surface coverage and peening intensity.
Surface coverage is a measure of the percentage of the article's surface area impacted by the shot particles. The cumulative impacts of the shot particles should nearly or fully saturate the surface area to obtain the mechanical enhancements normally sought through peening. Peening intensity is a measure of the amount of force of each individual impact and the cumulative work done on the surface of the article. Peening intensity is thus a function of the impact velocity and particle size and density (i.e., mass). The optimum peening intensity and resultant compressive stress layer depth is also related to the metal alloy and hardness of the material being peened.
However, in contrast to the typical applications of shot peening, the use of shot peening in the manufacture of textured press plates does not primarily seek enhancement of mechanical properties nearly as much as providing the desired press plate surface finish aesthetics, including its texture, gloss, and overall appearance while retaining its requisite flatness. Strict adherence to surface coverage and peening intensity criteria, as in mechanical applications of shot peening, are not critical except from the standpoint of consistently obtaining the desired textured plate and resultant laminate appearance. Maintaining plate flatness during peening and obtaining uniform gloss and roughness over the surface of the plate are important considerations.
Peened textured press plates typically begin as highly polished plates having a surface finish known in the press plate and laminate industry as a #7 mirror finish. These plates are predominantly fabricated from AISI 410 stainless steel. This alloy offers relative hardness, damage resistance, and corrosion resistance, as well as dimensional stability after repeated thermal cycling. Shot peening an initial #7 mirror plate finish has been found to inevitably lower the gloss of the plate. This results in a duller surface finish due to the myriad indentations which cause increased light scattering as perceived either visually or instrumentally (i.e., through use of an electronic gloss meter). In general, the greater the shot coverage up to saturation, the lower the resultant gloss, as the total area of shiny "islands" (nonimpacted areas) on the surface decreases. Importantly, saturation of the surface can be achieved relatively easily when smaller or finer shot grades are used, eliminating an overall speckled appearance and obtaining a uniformly dull "matte" (lightly textured) finish.
For aesthetic reasons, laminates with "rougher" textures to simulate a "suede" finish are sometimes desired. "Suede" textures have been found attainable through the use of larger shot grades and/or increased impact velocity, both of which as discussed above contribute to greater peening intensity. However, problems occur when larger shot grades are used.
In "rougher" textures obtained through the use of larger shot grades (creating greater impact crater depths through greater impact forces), courser peak structures appear. Further, since press plates are large, thin, and flat articles, they become more and more dimensionally unstable with greater peening intensity. For example, peening one side of the plate with a large shot grade at high velocities causes plates to warp with a convex curvature on the peened face. Although this can be somewhat solved by "balanced" shot peening machines which equalize face to face induced compressive stresses by simultaneously blasting both sides of the plate, larger shot grades still tend to heighten dimensional instability.
Additionally, with larger shot grades, it has been found progressively more difficult to achieve a saturated surface free of what is often referred to as objectionable "sparkle." Peening machines essentially process a given mass of shot per unit time. Therefore, the number of pellets striking a plate surface per unit time (i.e., surface coverage)is an inverse cubic function of increased shot diameters, necessitating either significant slowing down of the peening speed, multiple passes, propelling a greater mass of beads per unit time, or a combination of the above. The former two approaches have serious drawbacks from a practical commercial viewpoint, and the latter alternative is governed by machine design limitations. To achieve an acceptably uniform plate appearance approaching saturation with larger shot grades, all three alternatives in effect result in a greater peening intensity and associated stress relief problems if not perfectly balanced.
Moreover, producing fully saturated, roughly peened surface textures is not usually difficult to achieve in a laboratory or pilot plant environment. However, serious surface coverage and peening intensity restrictions contributing to overall roughness limitations are often encountered with large production size plates. These press plates must remain nearly perfectly flat after peening if they are to be used in a conventional high pressure decorative laminate manufacturing process, which use a plurality of plates during each pressing operation.
Thus, the process described above using a single size range or grade steel shot to texture a plate has serious limitations in its ability to achieve the higher roughnesses desired, which requires the use of larger shot grade, while still achieving adequate coverage and a uniform finish (without excessive sparkle) within reasonable processing times.
U.S. Pat. No. 4,076,566 largely resolved the above diametrically opposed roughness and coverage constraints imposed when processing full size production plates. It disclosed first using a relatively large shot grade to obtain the desired roughness and texture structure, followed by a second smaller grit or shot grade to achieve a fully saturated sparkle-free surface appearance. Although the desired macro-texture or roughness is first developed using large shot grade peening, full coverage does not result. The plate is then blasted with smaller shot grade (with much more efficient covering ability) to obtain saturation and elimination of residual high gloss islands without altering the initially formed macro-texture of the plate. Importantly, the need for subsequent electropolishing and preferred chroming is eliminated with this process.
Moreover, the use of steel shot in the peening process of the prior art causes several problems. Steel shot with the preferred hardness is brittle and prone to shatter. Softer steel alloys tend to flatten and cannot be readily separated from the shot peening machines. Moreover, each hardness of steel shot has a different effect on the roughness of the finished press plate. Steel is also quite dense, requiring powerful blasting machines which result in greater cost and machine wear.
Additionally, steel shot leaves an iron/iron oxide residue within the impact craters generated on the surface of the press plates. These deposits, particularly when exposed to moisture, quickly oxidize and result in an objectionable overall rusty or "blackened" appearance to the plate surface. The oxide residue is difficult to remove by normal washing techniques and often requires special treatment prior to any final processing (e.g., chrome plating) or laminate pressing.
Another conventional shot peening material is glass beads, particularly used to avoid the contamination associated with steel shot when processing stainless steel and non-ferrous metals. However, glass beads have three distinct disadvantages compared to other shot materials, particularly as applied to texturing or overblasting stainless steel press plates. First, being less dense than either steel or ceramic shot, it is difficult with glass shot to obtain blasting intensities required to achieve relatively deep texturing, even at high impact velocities. Second, glass beads are very brittle and are prone to breakage and shattering into a fine glass "dust", particularly at high impact velocities. This dust is very aggressive towards the plate surface, behaving in a manner similar to sharp angled "grit" rather than the preferred spherical shot, resulting in micro-scarring of the plate surface and undesirable light scattering and "grayness" of the surface of laminate pressed from it. In fact, glass beads shatter so quickly that during the processing of a single plate, a severe gloss gradient can develop along its length due to the progressive rise in abrasive blasting with an accompanying decrease in the preferred shot peening.
A final problem is the extreme hygroscopicity of glass beads, causing cohesion between the beads and sporadic or total interruption of the bead flow to the blast guns. This further promotes an unacceptably non-uniform blast pattern and gloss differential over the surface of a plate. Conversely, the flow fluidity of other shot materials, especially ceramic shot, are unaffected by ambient moisture conditions.
Even after the finished textured press plates are obtained in any of the methods discussed above, refinishing remains a large concern. After a number of pressings, the plate often develops microscratches induced by laminate surface materials, especially by the presence of hard grit particles which are now more frequently being added to the laminate surface to improve wear and mar-resistance. Also, macroscopic imperfections may occur during routine handling in the form of burnishing, rub marks and small scratches. These defects on the plate surface in turn tend to degrade the surface finish of the laminates, which faithfully reproduce the press plate surface finish. When the microscratches and visible defects become too pronounced, the plates must be replaced or refinished. Thus, the texturing process must also be capable of consistently providing a method to refinish the press plates.
As an aspect of the present invention, it has been recognized that the combined texture and gloss achieved by mixing different grades of shot provides equivalent or improved results compared to using a discrete shot grade and peening a plate several times, each peening operation using a different sized shot grade. Thus, mixing different sized shot grades is a superior method than shot peening a plate multiple times with different sized shot grades each time. The former method avoids the need for separate overblast steps. Furthermore, mixing various shot grades avoids having to empty the blasting machine of large shot grades and refilling the machine with small shot grades or the converse. This conversion process is time consuming, particularly if a large quantity of plates are to be processed. Mixing shot grades also avoids the alternative of dedicating equipment to each grade of shot used in the process to maintain productivity.
It has thus been discovered that the two-step process of large shot grade peening followed by small shot grade overblasting as disclosed in U.S. Pat. No. 4,076,566 can be simplified to a process having significantly fewer blasting operations while achieving essentially the same results, wherein a mixture of a predominantly large shot grade (preferably 75-90%) and a small shot grade (preferably 10-25%) can be used simultaneously to effectively texture the press plate and provide the full coverage and sparkle-free surface desired. In combination with the improvements sought, the further use of ceramic shot rather than conventional steel shot further avoids the need for chemical decontamination after peening and the disadvantages of glass shot.
Thus, the present invention contemplates using a mixture of at least a first shot grade and a second shot grade in the plate finishing process. Large shot grade peening and small shot grade overblasting are simplified to a one step process while achieving the same results as multiple passes with different shot grades. Furthermore, mixtures of various grades of "larger" shot may be used in that portion of the total mix to form a more visually complex and pleasing textured structure and pattern, wherein a wider variety of intermixed large shot grade impact crater diameters form a continuous three dimensional roughness matrix.
The further advantages of the use of mixed shot grade peening and ceramic shot will become apparent to those versed in the art, such that the above disclosures set forth should not be construed as limiting to the scope of the instant invention as claimed.